The results show that elevated [CO2] (450–800 ppm) significantly increased wheatgrain yield by 24% with 95% bootstrapped confidence intervals of 20–28% across all studies. The increasewas dependent on experimental conditions, such as CO2exposure method (44% less yield increase in free-air CO2enrichment than in enclosure studies), rooting system (smaller response in field compared to potexperiments) and possible coinciding stress factors (low nitrogen and drought).